How did my fish die so fast?

ok i got a brand new tank for christmas (14 gallon). i put water conditioner in it. rinsed off the gravel, bought two aquatic plants, and i put an air pump in it. (water temp was at 78 and the ph level was at 7.2). I just went to the pet warhouse and bought 4 fish. a Pineapple swordfish, Sail fin Molly, Dalmation molly, and an Opaline blue. lets just say t was like a domino death effect. two fish died after an hour, so i bought another pineapple. when i came home with the new fish, the original pineapple was dead. so now it was just the pineapple swordfish and the opaline blue. eventually they both died together. but every fish was fine until i removed the first dead fish. once the dalmation died, i scooped him out. immediately after, the sail fin died. an hour later the first pineapple died. then two hours later the remaining fish died together. but once i scooped the pineapple out, the opaline started the death dance and the second pineapple follwed. i even bought this bacteria stuff to help make the water more welcoming to new fish, and i put the fish over the tank and did that jazz before releasing them. if anyone can help me understand what went wrong and how i can improve my fish tank please respond to this question or email me at quackerjack4@yahoo.com thank you.